The background of the application is described in two parts:
1. Effects of working memory and cognitive load in difficult listening situations is reviewed
2. Hearing aid signal processing that may improve/ameliorate cognitive load is Reviewed
1. Effects of Working Memory and Cognitive Load in Difficult Listening Situations
In an optimum listening situation, the speech signal is processed effortlessly and automatically. This means that the cognitive processing involved is largely unconscious and implicit. However, listening conditions are often suboptimum, which means that implicit cognitive processes may be insufficient to unlock the meaning in the speech stream. Resolving ambiguities among previous speech elements and constructing expectations of prospective exchanges in the dialogue are examples of the complex processes that may arise. These processes are effortful and conscious and thus involve explicit cognitive processing.
Working memory (WM) capacity is relatively constant but varies between individuals (Engle et al., 1999). In performing dual tasks which tax the working memory, there are large individual differences in the ability to assign cognitive resources to both tasks (Li et al., 2001). It has yet to be investigated how persons with HI allocate their cognitive resources to different aspects of the language understanding process and how much cognitive spare capacity (CSC) remains to be devoted to other tasks once successful listening has been accomplished.
The ELU (Rönnberg, 2003; Rönnberg, Rudner, Foo & Lunner, 2008) relies on the quality of phonological representations in long-term memory, lexical access speed, and explicit storage and processing capacity in working memory. When phonological information extracted from the speech signal can be matched rapidly and smoothly in working memory to phonological representations in long term memory, cognitive processing is implicit and ELU is high. The ELU framework predicts that when mismatch occurs in a communicative situation, it not only elicits a measurable physiological response, it also leads to an engagement of explicit cognitive processes, such as comparison, manipulation and inference making. These processes engage explicit processing and short-term storage capacity in working memory, which can be termed complex working memory capacity. Thus, individual complex working memory capacity is crucial for compensating mismatch.
Listening situations with various background noises or reverberation makes the (speech) signal suboptimal and influence speech recognition both for normal hearing persons and hearing impaired persons but to different extent.
Results by Lunner and Sundewall-Thorén (2007) suggests that in an aided condition with slow-acting compression and unmodulated noise the test subjects' cognitive capacities are active, but without exceeding the capacity limit of most hearing impaired individual listeners. Thus, the individual peripheral hearing loss restrains the performance and the performance may be explained by audibility. Possession of greater cognitive capacity confers relatively little benefit. However, in the complex situation with fast-acting compression and varying background noise, much more cognitive capacity is required for successful listening. Thus, the individual cognitive capacity restrains the performance and the speech-in-noise performance may, at least partly, be explained from individual working memory capacity.
Furthermore, Sarampralis et al. (2008) have shown that the about 4 dB SNR improvement (attenuation of spatially separated disturbing sources) of directional microphones (in comparison to omnidirectional microphones) have implications for improved memory (recall) and faster response times. Sarampralis et al. (2008) have also shown positive results on memory (recall) and response times for noise reduction systems.
A hearing impairment will restrict the amount of information transferred to the brain as well the signal information being of poorer quality compared to normal hearing people because of the perceptual consequences of the cochlear damage, such as reduced time and frequency resolution, difficulties to utilize temporal fine-structure, worse ability for grouping of sound streams as well as worse abilities to segregate sound streams. Thus, for the hearing impaired more situations will provoke effortful explicit processing. For example, hearing impaired are more susceptible to reverberation, background noises, especially fluctuation noises or other talkers, as well as have worse abilities for spatial separation than normal hearing persons.
2. Hearing Aid Signal Processing that May Improve/Ameliorate Cognitive Load
Hearing aids have several purposes; first of all they compensate the reduced sensitivity for weak sounds as well as the abnormal growth of loudness through the use of multi-channel compression amplification systems, with either fast or slow time constants (Fast-acting compression can actually be seen as a noise-reduction system under certain conditions, see e.g. Naylor et al. (2006). In addition there are ‘helping systems’ that may reduce cognitive load that are used in certain situations to improve speech recognition in noise and under other circumstances to increase comfort when speech not is present. Edwards et al. (2007) have shown that directional microphones and noise reduction systems increase memory and reduce response times compared to the unprocessed cases, i.e. indications on less cognitive load. The main components of such helping systems are directional microphones and noise reduction systems. The helping systems are usually automatically invoked based on information from detectors, such as speech/no-speech detectors, signal-to-noise ratio detectors, front/back detectors, and level detectors. The underlying assumption is that the detectors can help to distinguish between ‘easier’ listening situations and more ‘difficult’/demanding situations. This information is used to automate the switching in-and out of the helping systems to help the user to have a comfortable monitoring sound processing when speech is not present to a more aggressive directional microphone set-up and noise reduction system when being in a demanding communication situation.
The ‘helping systems’ are only used in certain listening situations because they give benefit in only these situations, in other situations they may actually be contra-productive, for example invoking directional microphones, which attenuates sounds from other directions than the frontal direction, in a situation where there are little background noise and/or where information from behind are of importance, the directional microphones may actually worsen for example localization and probably be more effortful than a omni-directional microphone. Thus, the directional system may negatively influence naturalness, orientation abilities, and object formation, localization abilities.
Similar drawbacks are present for noise reduction systems.
U.S. Pat. No. 6,330,339 describes a hearing aid comprising means for detecting a condition of a wearer (biological information, motion) and means for determining a mode of operation of the hearing aid based on a predetermined algorithm. The condition detecting means use outputs of a pulse sensor, a brain wave sensor, a conductivity sensor and an acceleration sensor, respectively. By this, the characteristics of the hearing aid can be varied adapting to the wearer's condition.